The advantages of devices in which plasma is generated by electron cyclotron resonance excitation are that a highly active plasma can be generated at low gas pressures, ion energies can be chosen from a wide range of magnitudes, large ionic currents can be realized, excellent directivity and homogeneity of ionic currents can be achieved, etc. These are the reasons for the continued research and development of such devices which are considered indispensable in manufacturing high-density semiconductor elements and other devices.
FIG. 1 is a longitudinal section of a conventional microwave plasma processing device (e.g. see U.S. Pat. No. 4,401,054) which is intended to be used as an etching device and which operates on the principle of electron cyclotron resonance excitation initiated by microwaves.
As shown in FIG. 1, plasma generation chamber 31 has double peripheral walls forming a cooling water conduction chamber 31a, microwave lead-in opening 31c sealed with quartz glass plate 31b is in the center of the upper wall, and plasma withdrawal opening 31d is in the center of the lower wall opposite microwave lead-in opening 31c. Waveguide 32 has one end thereof connected to microwave lead-in opening 31c. Sample chamber 33 is positioned facing plasma withdrawal opening 31d, and exciting coils 34 are placed coaxially both with plasma generation chamber 31 and with waveguide 32 connected to it, while coils 34 enclose both chamber 31 and the end portion of waveguide 32.
Sample stage 37 is located opposite plasma withdrawal opening 31d inside sample chamber 33. A wafer or some other sample S is mounted on stage 37 by being placed in a simple manner on top of stage 37 or by being detachably mounted through the intermediary of electrostatic clamping or other conventional means. Furthermore, evacuation port 33a connected to an evacuation device (not shown) is furnished in the lower wall of sample chamber 33.
Gas supply system 31g is connected to plasma generation chamber 31 and gas supply system 33g is connected to sample chamber 33. Cooling water supply and drainage systems 31h, 31i circulate cooling water in chamber 31a.
In an etching device of this type, gas is supplied into plasma generation chamber 31 through gas supply system 31g after a required degree of vacuum is obtained both in plasma generation chamber 31 and in sample chamber 33, and a magnetic field is formed by exciting coils 34 while microwaves are introduced into plasma generation chamber 31 through microwave lead-in opening 31c, with plasma being formed as a result of resonance excitation initiated in the gas inside plasma generation chamber 31 which serves as a cavity resonator. A divergent magnetic field formed by exciting coils 34 and having a magnetic flux density which diminishes in the direction of sample chamber 33 projects the generated plasma into the area occupied by sample S in sample chamber 33, thereby ensuring that the surface of sample S inside sample chamber 33 is etched (see Japanese Laid-Open Patent Application No. 57-133636).
In the conventional device described above, microwave lead-in opening 31c in the upper wall of plasma generation chamber 31 is sealed airtight by quartz glass plate 31b which is penetrable by microwaves. Quartz glass plate 31b is additionally secured by fastener 32b which is arranged along the outer periphery of microwave lead-in opening 31c, sealing opening 31c and overlapping flange 32a at the end portion of waveguide 32.
Therefore, when plasma generation chamber 31 functions as a cavity resonator for microwaves, the inside portion of microwave lead-in opening 31c becomes an additional empty space which creates a sharp change in level with respect to the inner surface of plasma generation chamber 31, thereby causing an abnormal reflection of microwaves in this area and, therefore, impairing the homogeneity of plasma distribution. As a measure intended to overcome this disadvantage, the device shown in FIG. 2 has been proposed (see Japanese Laid-Open Patent Application No. 63-318099).
As shown in FIG. 2, microwave-penetrable substance 48 is inserted into microwave lead-in opening 31c (which opens into plasma generation chamber 31) in a manner ensuring that the substance, when it fills up the space, stays uniplanar with the inner surface of the plasma generation chamber. Other elements are essentially the same as shown in FIG. 1, with identical units being assigned identical numbers.
Microwave-penetrable substance 48 is composed of two components: (a) disk 48a which has a diameter and axial size roughly equal to those of microwave lead-in opening 31c and (b) circular flange 48b which is on top of disk 48a and has dimensions larger than those of the disk 48a. The lower surface of disk 48a is positioned in such a manner as to be uniplanar with the inner surface of plasma generation chamber 31, with disk 48a being tightly fitted into microwave-penetrable opening 31c and an O-ring being inserted along the outer between opening 31c and flange 48b.
Therefore, microwave lead-in opening 31c in the upper wall of plasma generation chamber 31 is filled tightly through the intermediary of microwave-penetrable substance 48, thereby excluding any anomalous reflection of microwaves. The result is the corresponding reduction in the reflection factor of microwaves and an increase in the homogeneity of generated plasma.
However, a drawback of such a conventional device is an insufficient homogeneity of plasma distribution which is the result of a difference in diameters between microwave lead-in opening 31c and plasma generation chamber 31, with prevention of anomalous reflection caused by this difference being insufficient, thereby generating a complex distribution of the microwave magnetic field.
U.S. Pat. No. 4,960,073 (37 Suzuki"), assigned to Anelva Corp. in Japan, discloses a conventional microwave plasma treatment apparatus wherein electron cyclotron resonance (ECR) is utilized to form a plasma which is used to carry out surface treatments of a substrate. The surface treatments include etching, thin film deposition and formation of thin film. The apparatus includes a plasma generating chamber separated from a reaction chamber by a quartz ring which forms a plasma extracting aperture. One or more solenoids are provided around the outer periphery of the plasma chamber for forming a magnetic field inside the plasma chamber. Gas is introduced into the plasma chamber through one or more conduits which open into the plasma through an upstream end wall of the plasma chamber opposite to the quartz ring which forms a downstream end of the plasma chamber. The plasma and reaction chambers are maintained at subatmospheric pressure by suitable vacuum pumping means. Plasma is generated in the plasma chamber by introducing gas into the plasma chamber, by activating the solenoids to produce the magnetic field in the plasma chamber and by introducing microwaves through a window in the upstream wall.
Suzuki provides an even-numbered plurality of auxiliary magnets between the periphery of the plasma chamber and the solenoid coils. The auxiliary magnets are symmetrically arranged circumferentially around the central axis of the plasma chamber such that the polarity of adjacent magnets is reversed and to form a strong magnetic field locally in the vicinity of the inner wall surface of the plasma chamber to heighten plasma density in the vicinity of the inner wall. According to Suzuki, plasma density is substantially more uniform from the central axis to the vicinity of the inner wall and more uniform ionic current density distribution can be obtained in the reaction chamber.
Suzuki's apparatus includes a microwave introducing window made of a dielectric such as quartz glass or ceramic material and having a uniform thickness. A horn-like portion is provided between a waveguide pipe and the window. In addition, a block for propagating microwaves is provided in the plasma chamber around the window, the block having a horn-like inner surface, the diameter of which increases along the central axis in a direction away from the window. In another arrangement, the microwave introducing window is replaced by a wineglass-shaped wall, i.e., a bell jar. The wall is open at a downstream end thereof and closed at an upstream end. The wall is of uniform thickness and of dielectric material. The open end of the dielectric wall abuts a quartz ring having a plasma extraction orifice therein and the closed end of the dielectric wall extends part way into a wide end of a horn joined to a wall between the auxiliary magnets and the dielectric wall.
U.S. Pat. No. 4,857,809 ("Torii"), assigned to Nippon Telegraph and Telephone in Japan, discloses a microwave ion source utilizing a microwave and a magnetic field, the microwave being introduced through a window having a multilayered structure of plates with different dielectric constants. Torii utilizes a magnetic circuit outside the plasma generating chamber to produce a magnetic field intensity at least near a microwave introducing window at a value higher than that needed to generate electron cyclotron resonance (ECR). As a result, a narrow high-intensity plasma mode is generated such that the plasma density is higher at the center region of the plasma generation chamber. Torii provides an ion extraction electrode system between the plasma and reaction chambers to extract an ion beam from the center region of the narrow high-density plasma. The multilayered window includes a main window sealing the waveguide from the plasma chamber and an auxiliary window in the plasma chamber to protect the main window from a back stream of electrons which could otherwise damage the main window. The main window can be quartz and the auxiliary window can be alumina or a double layer of alumina and boron nitride.
U.S. Pat. No. 4,987,346 ("Katzschner"), assigned to Leybold AG in Germany, discloses an apparatus for reactive ion beam etching or plasma deposition. Katzschner discloses that the apparatus can generate a plasma beam having an effective diameter of more than 200 mm and homogeneity of the particle current density of over 95%. The apparatus utilizes a combination of torus-shaped magnetic fields with the microwave coupling in via an E-sector, H-sector, pyramidal or conical horn radiator. One or two sets of annular sets of magnets can also be used and the plasma chamber comprises a quartz container to minimize plasma impurities. For instance, a cone-shaped horn radiator can extend between a microwave waveguide and the end face of the quartz container and two annular permanent magnets with reversed polarities can surround the quartz container with an annular iron yoke surrounding both magnets whereby the torus-shaped magnetic field is generated within the quartz container. An electromagnet surrounds the horn radiator to generate field lines parallel to the center axis of the horn. An extraction device including three extraction grids is located at the exit of the quartz container.
Other arrangements of microwave windows are shown in U.S. Pat. No. 4,414,488 ("Huffmann"), U.S. Pat. No. 4,409,520 ("Koike") and U.S. Pat. No. 4,393,333 ("Sakudo").